The production of beverage containers such as those used for carbonated beverages ia a highly refined art. Numerous improvements have been made to reduce weight while providing satisfactory performance. A major advance was the introduction of the two piece system, wherein a top lid or closure is joined to a portion that includes both the container sidewall and the container bottom. This latter portion is typically drawn and ironed from a single piece of aluminum or steel to create a thin-walled container with a bottom structurally capable of withstanding the pressures and stresses produced by the carbonated contents.
It has been found that seemingly minor changes in the geometry of the bottom profile of a drawn and ironed container permit a less metal to be used while still maintaining acceptable performance. For example, it has been found that an upwardly domed configuration for the center panel of a bottom profile provides improved results. U.S. Pat. Nos. 4,685,582 and 4,768,672 to Pulciani et al. are directed to improved domed bottom profiles. These patents represent the current state of the art profile. U.S. Pat. No. 4,620,434--Pulciano et al. also discloses upwardly domed bottom profiles that resist dome reversal and teaches that the dome should not be made too shallow or it will fail by reversing. This type of failure is related to the forces generated by the internal pressures of the carbonated contents of the container and is typically referred to as the "static dome reversal pressure," i.e., the maximum pressure below which the container will not fail.
Several attempts have been made to optimize static dome reversal pressure in upwardly domed bottom configurations. For example, U.S. Pat. No. 4,834,256--McMillin teaches that it is possible to make shallower domes that exhibit acceptable resistance to reversal under static pressure conditions. Also, U.S. Pat. No. 4,953,738--Stirbis et al. teaches that the strength of a domed container bottom may be improved by adding grooves and ribs. This patent discusses the problem that arises when configuring domes because, although deeper domes are stronger, they reduce the useful volume of the container, requiring a taller container that is more susceptible to sidewall buckling. Finally, U.S. Pat. No. 4,919,294--Kawamoto et al. discloses non-spherical domes based upon a catenary curve profile. This patent includes data comparing the disclosed catenary domes with the prior art to show improvements in "pressure proofness" which apparently relates to static resistance to dome reversal.
In addition to the load placed in the container bottom by static pressure generated by the contents, it has been found that the forced generated during handling also may cause a filled container to fail. It has therefore been suggested that container be made more "drop resistant." For example, U.S. Pat. No. 5,105,973--Jentzsch et al. discloses bottom profiles that purportedly exhibit improved drop resistance. This patent teaches that although it is known to decrease dome diameter and increase panel height to attain improved drop resistance, it is preferable to use a profile having an annular supporting portion formed between the dome and the inner leg of the chime over at least part of this circumference.
Clearly, the variations in the geometry of bottom profiles are almost limitless. Nevertheless, despite the novel geometries known in the prior art, significant challenges arise when certain geometries are produced on the massive scale of commercial beverage container construction. Additionally, the complexities of the bottom forming machinery typically employed has hindered the introduction of improved bottom profiles.
Those of skill in the metal container forming arts are well aware of the various techniques by which a blank is drawn and ironed into a thin sidewall container that has an integral bottom profile. Moreover, it is well known to perform secondary operations to add additional features to the drawn and ironed container. For example, a necking operation may be used to create a particular profile at the open end of the container in order to provide the proper size and shape for engaging a closure or container lid. Apparatus for performing such necking operations is disclosed in U.S. Pat. No. 3,687,098--Maytag. The technology described in this patent is used commercially in Bellvac can necking apparatus Models 210, 575 and 595, available from Bellvac Production Machinery (USA). The Maytag patent discloses that a neck of reduced diameter is formed by axially moving an unformed container into a stationary necking die. The cooperation between the surfaces of the necking die and a punch that slides inside the container results in the desired deformation. A variation of this system is disclosed in U.S. Pat. No. 4,723,430--Hahn. By including axially deformable members in a mandrel, other profiles are be introduced into the can end. As shown in the Hahn patent, the shape of the deformable members is chosen so that when compressed, they expand radially outwardly to form a full circle. A similar technique is shown in U.S. Pat. No. 4,599,123--Christensson for flaring the open end of a container/lid assembly during welding.
Certain improvements have been made with reference to bottom reforming equipment as well. For example, EPO Publication No. 482 581 A1 discloses apparatus for making bottom profiles such as those disclosed in the Jentzsch et al. patent referenced above. The apparatus uses either a roller or a swaging tool that reforms the inner leg. The roller apparatus is disclosed in several embodiments that are moved circumferentially around the inner wall. In the case of the swaging tool, a portion of the tool moves transversely and is urged into the sidewall, deforming it. In order to locate the bottom profile of a container with relation to the reforming tooling, both embodiments use specially formed housing that has a mating profile machined thereon.
Despite the improvements made in bottom profile geometry and in container forming equipment, a need still remains for methods and apparatus that will permit bottom profile forming/reforming in a reliable manner suitable for high volume production. It is therefore an object of this invention to provide improved bottom profile reforming techniques that permit a wide variety of bottom profiles to be implemented. A further object of the present invention is to provide such improved techniques in the form of modifications to existing container forming equipment. This latter object permits improvements to be readily incorporated into existing production lines using machinery known to be reliable and which is familiar to its operators.